Paper manufacture is generally carried out by producing a slurry of cellulosic fibers in water containing a variety of additives and subsequently removing most of the water to form a thin paper web. The structural integrity of the paper arises, in large part, from mechanical entanglement of the cellulosic fibers in the web and hydrogen bonding between fibers.
With paper intended for use as tissue and towel products such as facial tissue, bathroom tissue, paper towels, napkins and the like, the level of structural integrity arising from the paper-making process conflicts somewhat with the degree of perceived softness that is necessary for consumer acceptance of such products. The most common method of increasing the perceived softness of tissue and towel products is to “crepe” the paper by mechanically compacting paper in the machine direction. The creping action imparts a fine, rippled texture to the sheet, increases the bulk of the sheet, and results in improved softness and absorbency, as well as providing other changes in physical properties such as stretch, particularly when measured in the machine direction.
Generally, creping is accomplished by fixing the moist, cellulosic paper web to a rotating thermal drum commonly known as a Yankee dryer, by applying the web onto the surface of the dryer onto which has been sprayed an adhesive composition and often one or more release agent or composition, generally in the form of an aqueous solution, emulsion or dispersion. The surface of the Yankee dryer is continuously sprayed with the adhesive/release agent composition, and the cellulosic web is subsequently and continuously applied, and the paper dries rapidly from hot air impinging on the exposed side of the paper as well as conductive heat transfer from the drum. It is generally thought that as the paper dries, hydrogen bonds form between the fibers and a flat and dense web morphology is created. The web then is scraped backwardly upon itself and off of the Yankee dryer by means of a flexible blade, termed a doctor blade, a creping blade, or simply a creper. This creping process causes a substantial number of inter-fiber bonds to break, altering the physical-chemical characteristics of the web, and increasing the perceived softness of the resulting tissue and towel products.
The art of obtaining good crepe quality relies on obtaining and maintaining the proper level of adhesion between the paper web and the Yankee dryer. Inadequate adhesion can result in poor or non-existing creping or require lower speed operations due to slow drying, while excessive adhesion can lead to poor sheet quality or cause the sheet to break, interfering with stable operation. Webs which are insufficiently adhered to the Yankee dryer can impact the control of the web as it travels between the creping blade and the winder upon which a roll of the paper is being formed, causing problems in forming a uniform roll of paper. For example, a loose sheet between the creper and the roll can cause wrinkles, foldovers, or weaving of the edges of the sheet and the like in the rolled-up paper, adversely affecting subsequent operations of tissue and towel manufacture. Therefore, considerable effort has centered around adjusting the balance between adhesion and releasability of the web, and combining these balanced factors with the right combination of web dryness at the crepe blade, blade angle, and other conventional parameters that govern creping.
Conventional release agents or release aids can alter the properties of an adhesive polymer and further can provide lubrication to the doctor blade, moderate the adhesive properties of the adhesive coating, and influence the release of the paper web from the Yankee dryer, all of which can affect the properties of the paper product. Some release aids are non-polar, neutral and non-water soluble compounds. Cationic release aids are also available and comprise water insoluble materials, such as imidazoline quaternary salts. Thus, tissue and towel paper quality and production efficiency can depend in part on the interaction of a release aid with both paper and adhesive. Depending upon the particular release agent or composition, the release mechanism can be related to an ionic interaction, for example, a cationic release agent's interaction with anionic cellulose fibers, and/or be related to a phase separation that occurs. Release aids also may be non-ionic, for example, may be oil based and thereby be associated with very different release mechanism.
While many advances in creping adhesives have been made in recent years, developments in new and modified release aids have lagged somewhat. There is a continuing need for new and useful release aids and in particular, new combinations of adhesive compositions and release aids with properties that allow more fine control over the adhesion-release balance. Desirably, new release aids and combinations of adhesives and release aids should offer a better means to manage the creping process by allowing better adjustments in adhesion, tack and rewetability, and release properties between the cellulosic fiber web and the surface of the Yankee dryer from which the paper is creped.